I have four children. The two youngest are boys, aged 33 and 29. The elder is married and has a very young son. The younger is in a serious relationship.

When the older one got married, he treated his younger brother with anger and rudeness, even though the younger, as best man, knocked himself out to help and be supportive. When his wife got pregnant, he told us, his parents, and his older sister and brother straight away. When he did tell his younger brother the news, he made sure he knew he was the last to know. Doubtless this is just the tip of the iceberg, reflecting a power struggle between them and all sorts of other issues to which I am not party.

While they had their sibling issues growing up, they always managed to work things out. It is now more than a year since they spoke. The younger refuses to see the older unless he apologises, which he refuses to do.

Although my daughter, husband and I have tried to persuade them to let go of their grievances in the interests of the family, and they also had one meeting with an objective outsider, this has made no difference. How can this ugly impasse be broken?

Unusually this week, I have not had to edit the letter, so here it is in full. There isn't a huge amount to go on. What happened to trigger the not talking? What recurring themes come up when you talk to them about each other (which you obviously have but may wish to now stop, see later for my reasoning)?

Much sibling rivalry in normal trying-their-best families, is about perceived injustices, often relating back to childhood. Little, sometimes imperceptible, things can be taken into adulthood. Although of course, with four children, there will be four subjective views of their childhood.

I asked psychotherapist Judith Lask (aft.org.uk), who has experience in the field of sibling relationships, what you could do to move things along with your adult children: "The more you interfere, the worse it will be. You've made your point, you've got them outside help … your difficulty is that whatever you do [if you're not careful] it will be seen as taking sides."

This advice, by the way, is as much to you as your husband – Lask wonders what their dad made of it all.

Both she and I think this isn't a recent thing but hints at something underlying, which is why I asked you about childhood/recurrent themes.

Often, you see, children play out rejections they themselves have felt on younger or weaker siblings (and by children I also mean adult children). Lask wonders whether there had been any bullying of the younger one by the older one at any stage in their lives? Or, conversely, if the younger one had been favoured as the "baby"?

We could all postulate about why this has happened, but if there were any cracks, then any pressure on their relationship as adults may have caused that to widen. Was one son more reliant on the other as adults, and did their relationships with their wives/girlfriends make the other feel left out?

"Ultimately," says Lask, and whatever the reasons, "it needs both your younger sons to want to change things." She also wondered about their respective partners and if you knew what they thought of it all? What perspective do your other children bring to the situation?

It's amazing that they have managed to talk with an objective outsider, but remember that complicated matters may not be fixable with one meeting so that may be worth another go in the future.

In the meantime, what do you do now? "Make it clear that you realise it's hard for them and it's hard for you too," says Lask, "but be careful not to support the difference [between them]. So, continue to invite them both to family events – if they come or not is up to them. Don't tread on egg shells around them and don't divide them up over family events."

Lask also recommends not making too much of a fuss now as that could make it harder for them to step down over it. I asked what you could do if one or both brothers tried to embroil you in the situation about the other brother. "Listen a bit, but don't let the conversation get too far. It's a bit like when friends separate and you want to stay friends with both sides of the couple, you have to limit things." So that means not letting one slag off the other.

If one does start a diatribe about the other, Lask suggests saying something like "I know how you feel and if there's anything you think I can do …" so you are supportive without taking sides or letting them go on and on about it. Preserve a good relationship with each individually and, hopefully, in time they will find a way back to each other.